A conventional physical sensor includes a fixed section and a movable section supported by a beam structure on a substrate. The movable section moves relatively to the fixed section in accordance with application of a physical quantity such as an acceleration or an angular velocity. Using this mechanism, the physical sensor detects the applied physical quantity.
For example, in the method proposed in Patent Document 1, the movable section is formed as follows. An oxide-film sacrifice layer is sandwiched between upper and lower single-crystal layers for bonding. A groove is formed on the upper layer so as to reach the sacrifice layer. The sacrifice layer is etched through the groove. Similarly, Patent Document 2 discloses the multilayer laminate structure including multiple sacrifice layers. A beam structure is formed by removing the sacrifice layers.
Patent Document 3 proposes the technique to etch the surface of a beam or a movable section from the substrate surface by a prescribed quantity. The technique aims at adjusting the thickness of the beam or the movable section of a physical sensor that detects physical quantities applied from different directions on one semiconductor substrate. Patent Document 4 proposes the technique to configure a beam as follows. Hydrogen ion is injected at a position for forming a hollow region in the silicon. A heat process is then conducted to delaminate the position injected with the hydrogen and form a hollow region.
Patent Document 5 proposes the technique to control a torsion bar thickness as follows. An oxide film is embedded in a semiconductor substrate. The surface of the semiconductor substrate embedded with an insulating film is bonded to another semiconductor substrate. The oxide film embedded in the semiconductor substrate is used as a mask to etch the semiconductor substrate. A torsion bar is formed so as to be integral with the semiconductor substrate.
Patent Document 1: IP-A-6-349806 corresponding to U.S. Pat. No. 5,616,523
Patent Document 2: IP-A-2003-46091
Patent Document 3: IP-A-2004-93494
Patent Document 4: IP-A-2005-349533
Patent Document 5: IP-A-2004-106116 corresponding to US 2004/0053507
The above-mentioned physical sensor determines a sensor characteristic based on characteristics such as a spring constant for the beam. It is important to consider a beam size. For example, there may be a multi-axis sensor for detecting physical quantities at least in two directions in such a manner that a beam deflects parallel to and perpendicularly to the substrate surface. It is important to consider a spring constant for the beam dependent on the beam width in the direction parallel to the substrate surface and a spring constant and a beam height dependent on the beam thickness in the direction perpendicular to the substrate surface.
The techniques described in the above-mentioned patent documents inevitably determine the beam thickness and therefore the spring characteristic for the beam. Accordingly, the beam thickness is constant in the direction parallel to the substrate surface and in the direction perpendicular to the substrate surface. It is difficult to provide a structure that features an intended spring characteristic in each direction.